According to the prior art, the articles of the above referenced type are generally obtained by starting from two half-shells which are mechanically assembled by soldering or adhesive-bonding, with the hollow space defined inside the two half-shells being subsequently filled with foamed polyurethane.
According to another technique, disclosed in Italian patent application No. 21,815 A/87, a hollow case of thermoplastic polymer is formed by blow-molding and said hollow case is then filled with a polyurethane mixture, which is then foamed.
Foamed polyurethane, used as the heat-insulating material in both of the above described techniques, is obtained by starting from a formulation which is constituted by an organic diisocyanate, a polyol, a silicone surfactant, a polymerization catalyst and a foaming agent belonging to the class of chlorofluoroalkanes, such as FREON.RTM..
At present, the adoption of foamed polyurethane as a heat-insulating material causes problems of environmental character, in that the foaming agents of chlorofluoroalkane character, such as FREON.RTM., are regarded as one among the main causes of alteration and destruction of the ozone layer existing in the stratosphere.
Unfortunately, replacing polyurethane with an equivalent material is a problem not easily solved, in that this polymer, by being foamed in situ according to the well-known R.I.M. (Reaction Injection Molding) technique, makes it possible to obtain rigid, self-supporting structural elements even if the outer case is made from a thermoplastic polymer, and not from a metal sheet.
This desirable outcome is due to the fact that polyurethane, by reacting in situ, perfectly adheres to the inner walls of the outer case, thus forming one single structural body with them.